Method of heat treating hard anodized surfaces



United States Patent 3,488,262 METHOD OF HEAT TREATING HARD ANODIZED SURFACES Clarence W. Forestek, 8160 Briarwood Drive, Broadview Heights, Ohio No Drawing. Filed July 13, 1966, Ser. No. 564,738

Int. Cl. C23b /52 US. Cl. 204-37 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process of treating relatively thick anodized surfaces to make them resistant to thermal shock.

Anodizing is a process in which an aluminum oxide film is formed on the surface of aluminum or an aluminum alloy for protection against corrosion and for decorative effects. In anodizing, the part is made the anode and is immersed in an electrolytic bath such as sulfuric, oxalic or chromic acid through which direct current is passed. While the film formed by anodizing is characterized as an aluminum oxide film, it may also contain aluminum hydroxides (Al O -XH O).

Anodizing is unique in the sense that the film is formed from the outside of the surface inwardly instead of being built up or deposited on the surface as in a plating operation.

Hard anodizing is an anodizing process in which a relatively thick aluminum oxide surface is formed on the part, such surfaces being approximately .001" to .008" thick. They are known in the trade as Alumilite Nos. 225, 226, 725 or 726, Martin Hard Coat and Sanford Process Hard Coat.

Since this aluminum oxide layer is extremely hard and highly stressed, it is liable to craze and crack, particularly when subjected to thermal shock. When this happens, the serviceability and corrosion resistance of the surface is seriously affected.

The thinner decorative anodized surfaces are often sealed by immersing the part in hot water with a small amount (5 to of potassium dichromate or the like. This is known as hot water sealing. Thick hard anodized surfaces cannot be hot water sealed because they cannot withstand the thermal shock and for this reason it is generally specified that such surfaces cannot be heated for any purpose without causing serious crazing and cracking in the surface.

In accordance with my invention, I have discovered a process whereby a hard anodized surface may be treated to make it resistant to thermal shock without ill effects. Obviously, this broadens and enhances the commercial possibilities for the use of such surfaces.

The area in which I am especially concerned is the utilization and treatment of a hard anodized surface with perfluorocarbon particles as set out in my co-pending application Ser. No. 474,130, filed July 22, 1965, entitled Treating Surfaces with Perfluorocarbon Particles.

In the preferred form of my invention, I place the hard anodized part in an oven promptly after the anodizing treatment (but only after the part is dry) and slowly heat it to about 200 F. The minimum heating ice time is one-half hour. The heating should be primarily by convection. Non-uniform heating, such as by contact with hot metal surfaces, is to be avoided. It is not necessary to control the atmosphere in the heating oven. I can use either a gas or an electric oven. The parts must be thoroughly dry before heating. After the part has been brought up to the temperature of 200 F., the oven is turned off and the part is gradually cooled. Larger parts may be removed from the oven and cooled in the room atmosphere. Rapid cooling of the part is to be avoided. After this treatment, the part can be heated rapidly up to about 200 F. and subjected to thermal shock (i.e., rapid cooling) without ill effects.

I have discovered that after the initial heating, there seems to be a change in the way in which the surface responds to heating. Apparently, the internal stresses are relieved and dehydration is effected without crazing. Heating seems to make the surface slightly smoother and slightly harder.

In a less preferred form of the invention, the part can be heated slowly to F. and then maintained at that temperature for at least 20 minutes. It may be necessary, however, to maintain the part at that temperature for a longer period of time depending upon the size of the part and other variables which trial and error will quickly resolve. Thereafter, the part is treated as described above.

I can, of course, compromise between the two forms of the invention and, for example, hold the part at F. for at least 10 minutes. As already stated, however, I prefer to heat the part slowly up to 200 F. and then gradually cool it off. This eliminates a holding period at a lesser temperature.

While the part can be heated to temperatures above 200 F., one should be very careful in going through temperatures at which water of hydration may be explosively released, i.e., 212 F. and above. Explosive release of water of hydration is to be avoided.

When the part must be thermally stable over 200 F., I prefer to heat the part for a second time from room temperature to at least about 400 F. Such is preferably done slowly by which I mean in not less than 30 minutes. Thereafter, the part is cooled back to room temperature and will be found to be resistant to thermal shock up to the temperature at which it has been heated.

As stated, I prefer two beatings instead of one. I believe that whatever surface phenomena takes place does so to a major extent in the first heating up to 200 F. I feel that I can omit the second heating, but prefer not to do so because additional internal stresses are relieved in the second heating. In the first heating, I prefer to stay below 212 F. and thereby avoid explosive release of water of hydration.

EXAMPLE I A former part for fabricating paper and polyethylene bags was hard anodized in an Alumilite No. 225 system. The former was made out of a 6061 aluminum alloy.

The former was placed in an electric oven without any atmospheric control and gradually heated from room temperature to 200 F. It took 30 minutes to heat the former from room temperature to 200 F. At the end of 30 minutes, the oven was turned off and allowed to cool gradually back to room temperature. Then the former was again heated, this time up to 400 F. It was removed from the oven and promptly treated with finely divided Teflon particles at below room temperature in accordance with my co-pending application previously identified. This shock did not craze the surface at all. The former then was subjected to its regular use.

. 3 EXAMPLE 11 A wrought aluminum valve part for an aircraft engine made of 7075 alloy was hard anodized in accordance with Alumilite No. 225 process as before.

It was placed in a gas oven and the temperature was slowly raised to 200 F. It took approximately 30 minutes for the oven to reach 200 F. The valve part was placed on an asbestos board so that it would be heated evenly. After the part reached 200 B, it was removed.

from the oven and cooled in the room atmosphere.

As a check, it was again heated to 400 F. and then cooled. Thereafter, it was heated and reheated in use without any cracking or crazing.

Having thus described my invention, what I claim is:

1. A method of treating a hard anodized surface to make it resistant to thermal shock comprising the steps of anodizing an aluminum surface, thoroughly drying the surface at room temperature and then uniformly and gradually heating it up over a periodof not less than 30 minutes to at least 150 F. but not more than 212 F. and holding it at that temperature for a period of minutes ranging ininverse proportion from 20 to 150 F.

4 to approaching 0 at 200 F. or higher and then slowly cooling the surface back to room temperature.

2. A method of treating a hard anodized surface to make it resistant to thermal shock comprising the steps of anodizing an aluminum surface, thoroughly drying the surface at room temperature, gradually and uniformly heating it up to about 200 F. in not less than about 30 minutes, gradually cooling it back to room temperature, slowly heating it up to at least about 400 F. and then cooling it back to room temperature.

References Cited UNITED STATES 'PATENTS 2,174,840 10/1939 Robinson et a1 175-41.5 2,851,626 9/1958 Allard et a1. 20437 XR 2,943,031 6/1960 Wainer 20437 1 FOREIGN PATENTS 595,365 12/1947 Great Britain. 601,636 5/1948 Great Britain.

IOHNH. MACK, Primary Examiner W. B. VANSISE, Assistant Examiner 

